Example: I run into a Thing of Swirling dust,
What are the chances that I'll get a Stone? ( Ex. Water stone Fire stone )
What are the chances that I'll get a Gem? ( Ex. Fire Gem Fighting Gem )
What are the chances that I'll run into a Pokemon? ( Ex. Drilbur, or Excadrill )

However, you also have a chance of encountering Drilbur and Excadrill. I don't know the chances, but if we treat it as an outcome that is just as likely as an item (I slightly doubt this is the case) then:

I'm inclined to say that encounter of Drilbur is between 30 and 50%, but I'll just leave it as 3.7% as it'll just drop the stone chance by 3 to 11% and specific item chance by 1% if I did calculations correctly.